Isotope ratio analysis is used to measure the relative abundance of isotopes (isotope ratio) in a gaseous sample for example, the stable isotopic composition of oxygen and carbon (18O/16O and 13C/12C) is an important proxy indicator of paleoenvironmental changes recorded in carbonate minerals deposited for example, as marine sediments.
Isotope ratio mass spectrometry (IRMS) is a well-established technique for such analysis. IRMS offers relatively high throughput (several minutes of analysis time per sample) as well as high precision isotope measurements. However high precision is accomplished at the expense of flexibility; IRMS instruments accept analytes in the form of a relatively limited number of gases which must be isotopically representative of the original sample. One of the challenges in IRMS is to ensure that fractionation (i.e., a shift in the relative quantities of isotopes between the original sample analyte and the mass spectrometer) is minimized or prevented.
The two most common types of IRMS instruments are continuous flow and dual inlet. In dual inlet IRMS, purified gas obtained from a sample is alternated rapidly with a standard gas of known isotopic composition by means of a system of valves, so that a number of comparison measurements are made of both gases. In continuous flow IRMS, sample preparation occurs immediately before introduction to the IRMS, and the purified gas produced by the sample is measured just once. The standard gas may be measured before and after the sample, or after a series of sample measurements.
Whilst continuous flow IRMS instruments can achieve higher sample throughput and are more convenient to use than dual inlet instruments, the yielded data are of lower precision. A general review of IRMS and gas inlet systems for these may be found in Brenna et al, Mass Spectrometry Reviews, 1997, 16, p. 227-258.
IRMS is not, however, without disadvantages. It is not compatible with condensable gases or a sticky molecule such as water. If a mixture of gases is applied to the analyser, there is the danger of interferences by reactions within the ion source. Thus, generally, chemical preparation of the sample is necessary to transfer the isotope of interest to a molecule that is more easily analyzed, and to separate the sample from other gas molecules. Typically, though, the required steps of chemical conversion are time consuming, and may compromise overall accuracy and throughput. Moreover, in general terms, IRMS instruments tend to be expensive, voluminous, heavy, confined to a laboratory, and usually in need of a skilled operator.
Isotope ratio optical (usually infrared) spectrometry (IROS) is a more recently developed technique for isotope ratio analysis. Here, photo absorption by H2O molecules is measured and the isotopologies of H2O are calculated by spectroscopy. IROS has a number of benefits over IRMS, such as ease of use, cost and potential field portability. It also permits direct analysis of water, where IRMS requires initial conversion e.g. to H2 or CO2, or equilibration with CO2, followed by analysis in gaseous form.
Relative to IRMS however, IROS typically offers a smaller dynamic range, poorer linearity, a larger measurement cell volume and pressure, such that more sample is required, and a higher pressure in the analyte cell.
Nevertheless, the gas load of an IR spectrometer may be very high. Thus, dilution of the sample to a relatively low concentration is often not a disadvantage. Indeed, given the limited dynamic range of an IROS instrument, significant dilution of the sample might be mandatory. The use of dry air as a carrier gas in IROS presents a further challenge relative to IRMS (where, typically, Helium is used as a carrier gas instead). The diffusion coefficient of CO2 in air is 0.16 cm2/sec, compared with a diffusion coefficient of about 0.7 cm2/sec for CO2 in Helium. Thus CO2 mixes much more slowly (by a factor greater than 4) in an IROS instrument than in an IRMS device, such that, in an IROS instrument, mixing is a more challenging issue.
For the sample quantities required for infrared laser spectroscopy, the major part of the substance has to be transferred into the laser cell, for example by the use of a carrier gas. However, the transfer of the substance into the laser cell without fractionation (i.e. modification of the isotope signature) is demanding. In IRMS, fractionation can be avoided either by transferring only a small part of the substance, thus not disturbing the chemical equilibrium, or transferring (and measuring) substantially the whole sample. Different parts of the transferred sample may have different isotope signatures but the whole time dependent peak is used, and differences in isotope fractionation cancel one another out during integration. The same principle applies even if, a constant fraction of the sample is split away, along as that fraction is time constant, that is, the proportion of sample which is split away remains constant over the measurement period.
Unfortunately, neither principle is possible for IROS. It is not possible to transfer only a small part of the substance for IROS because most or all of the total substance is required for analysis. Likewise transferring the whole sample and relying upon integration of the whole time dependent peak is not possible for IROS either. The signal in the laser cell has to remain as constant as possible. A conventional “peak shaped” transient signal typically has a relatively small start and end part and these cannot be evaluated. These initial and final parts of the transient signal often differ in isotope distribution from the rest and it is therefore necessary to have these integrated into the whole sample result.
When the majority of the sample is transferred out of the closed volume by a carrier gas, diffusion occurs at the boundary carrier gas/analyte, which is what leads to fractionation. FIG. 1 shows an example of such fractionation: on the vertical axis, the isotope ratio is plotted in the form of a “delta” value, whilst the horizontal axis represents time. The left hand plot in FIG. 1 represents experimentally obtained data, whilst the right hand plot exhibits a theoretical calculation. In each case, fractionation is clearly visible.
Against this background, the present invention seeks to provide a gas inlet system for an isotope ratio spectrometer that allows an improved supply of analyte gas to the spectrometric analyzer. Although the present invention seeks in particular to address at least some of the challenges presented by an IROS instrument, it is nevertheless also concerned with improving the manner of supply of analyte gas to an IRMS device as well.